1. Field of Invention
The field of the present invention relates in general to X-DSL communication devices and more particularly to a method and apparatus for channel estimation and fault detection in X-DSL communication systems.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including, G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc.
One of the factors limiting the setup and operation of X-DSL communication systems is channel quality. Not all communications mediums, e.g. subscriber lines are capable of supporting various of the X-DSL protocols. On a subscriber line the presence of bridges, taps, isolators, filters etc. may effect channel quality to the point where a given protocol may not be supported. Alternately, in operation channel quality may degrade due to improper repair or maintenance of the subscriber line. In each instance the typical solution is to decouple the subscriber line in the frame room of the PSTN central office and to couple it to test equipment. The test equipment typically injects an impulse into the line and measures the amplitude and delay of each of the resultant echoes or reflections generated by the line. The pulse typically has a duration shorter than the delay interval between any of the reflections in order to distinguish one reflection from the other. The energy injected into the line by the pulse determines the accuracy and completeness of the channel estimation produced thereby. A number of factors, however limit the energy of the pulse; i.e. its duration, subscriber line voltage/current limits, and the need to minimize interference with adjacent subscriber lines to which service is being provided.
What is needed are approaches to line estimation, qualification and fault detection that are lower in cost and which permit automation.